1. Field of the Invention
The invention relates to an improved rotor for an electrical machine, in particular for an electric motor.
2. Description of the Prior Art
It is known for rotors, for instance for brushless DC motors (BLDC motors), to be embodied with short-circuit element, on whose circumference permanent magnets are disposed. The short-circuit element can be assembled as a lamination packet made of laminations. For fastening purposes, it is known to glue the permanent magnets to the short-circuit element or to pot the short-circuit element and the permanent magnets with a synthetic resin or spray-coat them with plastic. Potting with synthetic resin is a kind of adhesive bonding. It is also known to insert the short-circuit element with the permanent magnets into a guard tube. A common feature of all these options is that the permanent magnets are positioned on the short-circuit element and must be retained during the curing of the adhesive, synthetic resin or plastic or during the insertion into the guard tube. In injection molding, holders must be provided in an injection tool that keep the permanent magnets positioned on the short-circuit element during the injection operation. In potting, the permanent magnets, which have tolerances, are placed in recesses of the short-circuit element that also have tolerances and are consequently potted in undefined positions.
One problem in positioning the permanent magnets on the short-circuit element is the production tolerance of the permanent magnets. Since the production tolerances, as dimensions become smaller, do not decrease to the same extent as the dimensions themselves, relative inaccuracies increase as the size of the permanent magnets and rotors decreases. The term “relative inaccuracy” means a tolerance with respect to absolute component dimensions. Particularly when rare earth magnets are used, which have a magnetic force multiple times higher than typical ferrite magnets and can therefore be smaller, an inaccuracy in positioning makes itself felt, because of the small absolute dimensions of the magnets and rotor.
Inaccurate positioning of the permanent magnets of a rotor makes itself felt in unequal so-called resting moments, which are dependent on an angular position of the rotor. In operation of the electric motor, an increased moment waviness and running behavior that is dependent on the direction of rotation can occur.